1. Field of the Invention
The present invention relates to a testing method of writing performance in a thin-film magnetic head having an electromagnetic transducer for writing data onto a magnetic recording medium. Further, the invention relates to a method for investigating behavior of magnetic domains in this electromagnetic transducer. Furthermore, the invention relates to a manufacturing method of a thin-film magnetic head for selecting a head using this testing method.
2. Description of the Related Art
Recently, there has been a growing demand for improvement of read/write characteristics of a thin-film magnetic head to achieve improvement of surface recording density in a magnetic recording/reproducing device, particularly in a hard disk drive. Regarding the read characteristic, development of magnetoresistive (MR) elements, using giant magnetoresistive (GMR) effect or tunnel magnetoresistive (TMR) effect, have been actively promoted. The MR element has been actually utilized as a read head element, and contributed remarkably to improvement of the surface recording density.
On the other hand, for the write characteristic, it has been a pressing need to improve the performance of a conventionally used electromagnetic transducer having a coil and a magnetic core. One of serious problems for the improvement is reduction of inductance of the electromagnetic transducer. Particularly, in order to improve the surface recording density, it is necessary to use a higher frequency of write current flowing through a write coil, and corresponding to this necessity, the inductance has to be small enough. As an example in which reduction of inductance has been achieved, Japanese Patent Publication No. 2000-207710A discloses a thin-film magnetic head. In this patent, a write coil layer is formed on a planarized surface with a lower core layer formed in a small size, whereby the write coil layer is formed properly and the inductance is reduced at the same time.
Japanese Patent Publication No. 03-132911A discloses a method. In this document, inductance of a write coil layer in a thin-film magnetic head is measured in both cases that a strong magnetic field is applied and not applied, respectively, and it is determined based on a difference between the measured values whether the head is good or not. For applying this method, a means of generating the strong magnetic field is necessitated. Moreover, Japanese Patent Publication No. 09-180138A discloses a testing method of an MR element to achieve accuracy of determination as to whether the MR element is good or not, by obtaining resistance vs. magnetic field characteristics of the MR element while write current is supplied to the electromagnetic transducer.
However, even if above-mentioned technologies are used, it has been difficult in the past to solve another important problem for improving performance of an electromagnetic transducer, that is, to control magnetic domains of a magnetic core. Actually, even in heads manufactured under the same manufacturing method with the same specification, some heads had a defect associated with the magnetic domains of the magnetic core, which has caused a reduced yield. The inventors of the present invention wrote data on a magnetic recording medium using electromagnetic transducers, and measured each bit error rate (BER) when the written data were read using an MR element. As a result, we found that a head, having a higher BER than a determination reference level, had a lowered writing performance and appeared to have abnormality in behavior of magnetic domains of the magnetic core at the time of the measurement.
However, it is very difficult to observe the magnetic domains of a magnetic core in a manufacturing process of a head because the observation requires spending a lot of time and labor. Accordingly, the determination as to whether the head is good or not, associated with the BER described above, can be made only when the head is finally mounted on a suspension to assemble a head gimbal assembly (HGA) and writing on a magnetic recording medium is performed. This means that a whole HGA, in which a defective head is incorporated, is disposed to thereby largely reduce the yield of the whole HGA. Moreover, such determination itself spends a lot of time and labor. Furthermore, this increases disposal of value-added HGAs.